Semiconductor workpieces are often implanted with dopant species to create a desired conductivity. The amount of dopant introduced into the workpiece is critical to its proper operation. Therefore, various techniques have been used to attempt to accurately measure the ion beam current supplied by an ion implantation system. In some embodiments, a charge collector, such as a Faraday cup, is positioned near the workpiece. In this way, the ion beam current can be measured based on the amount of charge collected by the Faraday cup over a given time period. A charge collector which is positioned relative to the workpiece may only receive charge from a portion of the ion beam. In other words, due to its fixed position, the charge collected may not representative of the entire ion beam.
In other embodiments, a charge collector may be mobile so as to be moved through the ion beam. In this case, a plate having an aperture may be disposed between the source of the ion beam and the charge collector, so as to limit the portion of the ion beam that is visible to the charge collector. As the plate and the charge collector are moved across the ion beam, the total charge can be integrated to calculate the beam current. This calculation is based on the speed at which the charge collector is moved and the size of the aperture. While this is a generally effective method to measure ion beam current, it may be susceptible to measurement error. For example, over time, the ion beam tends to erode the plate, particularly around the aperture, so as to increase its size. This increase in aperture size allows more ions to pass through to the charge collector. This results in the system calculating an ion beam current that is greater than the actual current. One method to address this issue is to replace the plate at a predetermined time interval before the effects of erosion become significant. However, this requires that the ion implantation system be taken off-line, thereby reducing efficiency and throughput.
Therefore, a system and method for detecting, and optionally correcting for, erosion of the aperture caused by the ion beam would be advantageous.